What exactly do you mean? Is it just that the indicator light is not on? What is the temperature of the water at the moment, and what it is usually?
If the water temperature is a lot lower than usual, yes the heater is broken and you need a new one.
If I remember correctly from another thread you are in the UK. If that's right, with the current heatwave the heater light won't come on at all because the air and water temperatures are higher than the heater is set for. If the water temperature is the same as usual, it is just the weather and the heater will turn on when it gets cold again.
If you don't have a thermometer, I recommend to get one - not the stick on the outside ones that change colour but one with liquid inside that goes inside the tank. And don't forget that because of the heatwave, the water will feel cooler than usual to your hand simply because you are warmer than usual.